A nonaqueous electrolyte secondary battery, especially a lithium ion secondary battery, has high energy density, and has been used in household compact equipment such as a cell phone or a personal computer. In recent years, application to automobiles has also been accelerated, in addition to the compact equipment.
The nonaqueous electrolyte secondary battery is a battery using an organic solvent-based electrolytic solution, and usually includes a positive electrode and a negative electrode, and also includes a separator disposed for the purpose of electrically insulating between these electrode plates. As the separator, for example, a microporous sheet made of a polyolefin-based resin is used.
Regarding the separator made of the microporous sheet, if high temperature breaks out inside the battery due to some abnormality, a shutdown function possessed by the separator leads to clogging of holes of the separator, and thus the conductivity inside the battery is lost and a battery function is lost. In such manner, the separator plays a role of maintaining safety of the nonaqueous electrolyte secondary battery. However, if the temperature of the battery exceeds, for example 150° C. due to momentarily generated heat, the separator may quickly undergo contraction to cause intense short circuit between the positive electrode and the negative electrode. In this case, the temperature of the battery may be abnormally raised to several hundred degrees (° C.) or higher to cause a problem in safety aspects.
As a study for improving the heat resistance of the nonaqueous electrolyte secondary battery, for example, Patent Document 1 proposes the technique in which an alumina porous film containing alumina is formed on a surface of any one of a positive electrode, a negative electrode and a separator that constitute a lithium ion secondary battery. It is considered that the alumina porous film has high heat resistance, thereby suppressing quick contraction of the separator.
However, it has been known that, in the battery using the alumina porous film, a trace amount of hydrogen fluoride existing in the battery reacts with alumina, thereby fluorinating a surface of the alumina to produce water. The water reacts with an electrolytic solution to cause the decomposition of the electrolytic solution, leading to the generation of a gas. Therefore, the durability of the battery such as cycle characteristic and storage characteristic may be degraded. The generation of a gas may cause not only degradation of the durability of the battery but also expansion and breakage of the battery, which may cause degradation of the safety.
As mentioned above, while high heat resistance can be obtained by using alumina, there is a problem that a gas is generated due to alumina to cause the degradations of the durability and the safety of the battery. Thus, there has been required the technique that can suppress the decomposition of an electrolytic solution to reduce the generation of a gas (hereinafter, “little gas generation” is sometimes referred to as “excellent electrolytic solution stability”) even when alumina is used to impart high heat resistance.